DISSECTION OF THE HEAD AND NECK

 

The study of head and neck anatomy provides a considerable intellectual challenge because the region is packed with small, important structures. These structures are associated with the proximal ends of the respiratory and gastrointestinal systems, the cranial nerves, and the organs of special sense. Dissection of the head and neck provides a special problem in that peripheral structures must be dissected long before their parent structure can be identified. A complete understanding of the region cannot be gained until the final dissection is completed.

 

TABLE OF CONTENTS

 

1.          SURFACE ANATOMY OF THE SKULL

2.          MONDAY, MARCH 22  – ANTERIOR ASPECT OF SKULL AND FACE

3.          TUESDAY, MARCH 23 – FINISH FACE; SCALP

4.          WEDNESDAY, MARCH 24 – INTERIOR OF SKULL; MENINGES OF BRAIN

5.          MONDAY, MARCH 29 – REMOVAL OF THE BRAIN AND THE CRANIAL FOSSAE

6.          THURSDAY, APRIL 8; FRIDAY, APRIL 9 – ORBIT AND CONTENTS

 

Head

The dissection of the head is foremost a dissection of the course and distribution of the cranial nerves and the branches of the external carotid artery. All of the cranial nerves and many blood vessels pass through openings in the skull. Therefore, the skull is an important tool with which to organize the study of the soft tissues of the head and neck. Parts of the skull will be studied as needed and details will be added as the dissection proceeds.

 

 

 

 

SURFACE ANATOMY OF THE SKULL

 

All parts of the skull are fragile, but the bones of the orbit are exceptionally delicate. The medial wall of the orbit is very easily broken. Never hold a skull by placing your fingers into the orbits.

  

Examine the skull from an anterior view and identify (Fig. 7.14):

Parts of several bones combine to form the following features (Fig. 7.14):

Lateral View of the Skull

Examine the skull from a lateral view and identify (Fig. 7.15):

 

Superior View of the Skull

The calvaria is the skull cap that is formed by parts of the frontal, parietal, and occipital bones. Examine the external surface of the calvaria and identify (Fig. 7.17):

 

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MONDAY, MARCH 22  – ANTERIOR ASPECT OF SKULL AND FACE

 

Face

Upon completion of this session, the student will be able to:

1.     Describe the location of the parotid salivary gland.

2.     Identify three main neurovascular structures that traverse the gland: the facial nerve, the retromandibular vein and external carotid artery.

3.     Identify the branches of the facial nerve in the face.

4.     Identify some exemplary muscles of facial expression.

5.     Trace the course of the facial artery and facial vein in the face.

 

 

Surface Anatomy

Palpate the following structures on the head of the cadaver (Fig. 7.18):

 

Dissection Overview

The skin of the face receives sensory innervation from three divisions (branches) of the trigeminal nerve (V). Two cervical spinal nerves complete the cutaneous innervation of the head (Fig. 7.19).

In contrast, all of the muscles of facial expression receive motor innervation from the facial nerve (VII).

The order of dissection will be as follows: The skin of the face will be removed to expose the muscles of facial expression. The parotid duct and gland will be identified. Branches of the facial nerve will be identified as they emerge from the anterior border of the parotid gland. Several facial muscles will be identified. Two important sphincter muscles will receive particular attention: the orbicularis oris (mouth) and the orbicularis oculi (eye). The terminal branches of the three divisions of the trigeminal nerve will be exposed where they emerge from openings in the skull.

 

 

Dissection Instructions

Skin Incisions

The skin of the face is very thin. It is firmly attached to the cartilage of the nose and ears but it is mobile over other parts of the face. This mobility permits the action of the muscles of facial expression. The muscles of facial expression are attached to the skin superficially and the bones of the skull deeply. They act as sphincters and dilators for the openings of the eyes, mouth, and nostrils.

 

  1. Place the cadaver in the supine position and refer to the skin incisions indicated in Figure 7.20.
  2. Remember that the skin of the face is thin. REMOVE ONLY THE SKIN. The muscles of facial expression insert into the skin and the branches of the facial nerve and vessels are very superficial.
  3. You do not need to make flaps as you do your skinning. You can remove the skin piecemeal and keep the dissected face covered with a moist towel.
  4. In the midline, make a skin incision that begins on the forehead at about the level of the hairline (A) passes through the nasion (B) and continues to the mental protuberance (C). Encircle the mouth at the margin of the lips.
  5. On the lateral surface of the head, make a skin incision from point A to the upper part of the ear, then passing anterior to the ear down to the level of the ear lobe.
  6. Starting at the nasion (B), make an incision that encircles the orbital margin. Extend the incision from the lateral angle of the eye to the incision near the ear.
  7. Make an incision from the mental protuberance (C) just inferior to the inferior border of the mandible to point D.
  8. Beginning at the midline, remove the skin of the forehead. Note that the skin adheres to tough subcutaneous connective tissue. Leave this connective tissue intact and do not remove the frontalis muscle.
  9. Remove the skin of the lower face, beginning at the midline and proceeding laterally. The superficial fascia of the face is thick and contains the muscles of facial expression.
  10. Detach the skin along the incision line from the forehead to the angle of the mandible (A to D).

 

Superficial Fascia of the Face

The superficial fascia of the face contains the parotid gland, part of the submandibular gland, muscles of facial expression, branches of the facial nerve, branches of the trigeminal nerve, and the facial artery and vein. The muscles of facial expression are attached to the skin, and these attachments have been severed during skin removal. The goal of this stage of the dissection is to identify some of the muscles of facial expression and follow branches of the facial nerve posteriorly into the parotid gland.

 

  1. A small part of the platysma muscle extends into the face along the inferior border of the mandible (Fig. 7.21). Recall that the inferior attachment of the platysma muscle is the superficial fascia of the upper thorax and that it forms a sheet of muscle that covers the anterior neck. Use blunt dissection to define the superior attachment of the muscle on the inferior border of the mandible, skin of the cheek, and angle of the mouth.
  2. Identify the masseter muscle. It is a thick muscle of mastication, which will be dissected later.
  3. Identify the parotid duct (Fig. 7.21). The parotid duct is approximately the diameter of a probe handle and it crosses the lateral surface of the masseter muscle about 2 cm inferior to the zygomatic arch. Use blunt dissection to follow the parotid duct anteriorly as far as the anterior border of the masseter muscle where the duct turns deeply, pierces the buccinator muscle of the cheek, and drains into the oral cavity.

Facial Nerve

  1. Use a probe to follow the parotid duct posteriorly and identify the anterior margin of the parotid gland.
  2. Study the branches of the facial nerve in an illustration (Fig. 7.21). Note that several small branches course parallel to the parotid duct. Using blunt dissection, locate one of these branches superior or inferior to the parotid duct.
  1. Use a probe to follow the branch toward the anterior margin of the parotid gland. Move the probe parallel to the branch as you dissect through the superficial fascia.
  2. At the anterior border of the parotid gland, extend your dissection field superiorly and inferiorly to locate other branches of the facial nerve. Identify the following:
  3. The parotid gland has very tough connective tissue that will not yield to a probe. To follow the branches of the facial nerve into the parotid gland, use the point of a scalpel blade as a probe.
  4. ON ONE SIDE ONLY, follow the branches of the facial nerve into the parotid gland. Superficial to the nerves, remove the parotid gland piece by piece. Within the parotid gland the nerve branches join to form the parotid plexus. Follow the nerve branches posteriorly until they combine into one or two nerves, just anterior to the ear lobe.
  5. Use a probe to define the anterior border of the masseter muscle. Anterior to the masseter muscle is the buccal fat pad. Use blunt dissection to remove the buccal fat pad and expose the buccinator muscle. Verify that the parotid duct pierces the buccinator muscle.
  6. Observe that two nerves enter the buccinator muscle:

Facial Artery and Vein

The facial artery and vein follow a winding course across the face and they may pass either superficial or deep to the muscles of facial expression.

  1. Find the facial artery where it crosses the inferior border of the mandible at the anterior edge of the masseter muscle (Fig. 7.21). The facial vein should be located posterior to the facial artery. At this location, the facial artery and vein are covered only by skin and the platysma muscle.
  2. Preserve the facial vessels. Reflect the platysma muscle by cutting it from the inferior border of the mandible and detach it from the angle of the mouth. NOTE THIS MUSCLE IS THIN AND FRIABLE.
  3. Use a probe to trace the facial artery superiorly toward the angle of the mouth. Observe that the facial artery has several loops or bends in this part of its course. Near the angle of the mouth, the facial artery gives off the inferior labial and superior labial arteries.
  4. Continue to trace the facial artery as far as the lateral side of the nose, where its name changes to angular artery.
  5. The facial vein receives tributaries that correspond to the branches of the facial artery. The angular vein has a clinically important anastomotic connection with the ophthalmic veins in the orbit, which will be detailed when the orbit is dissected.

Muscles Around the Orbital Opening

  1. At only 1 to 2 mm in thickness, the skin of the eyelids is the thinnest skin in the body. Carefully skin the upper and lower eyelids.
  2. Identify the orbicularis oculi muscle, which encircles the palpebral fissure (opening of the eyelid) (Fig. 7.22). Identify:
  3. Note that the medial attachment of the orbicularis oculi muscle is the medial orbital margin, the medial palpebral ligament, and the lacrimal bone. The lateral attachment of the orbicularis oculi muscle is the skin around the orbital margin. It is innervated by the temporal and zygomatic branches of the facial nerve.

Muscles Around the Oral Opening

  1. Several muscles alter the shape of the mouth and lips. Use a probe to define some of these muscles (Fig. 7.22):
  2. The muscles described above are innervated by the zygomatic, buccal, and mandibular branches of the facial nerve.

IN THE CLINIC: Facial Nerve

Bell's palsy is a sudden loss of control of the muscles of facial expression on one side of the face. The patient presents with drooping of the mouth and inability to close the eyelid on the affected side.

Sensory Nerves of the Face

  1. Use an illustration to summarize the sensory nerves of the face (Fig. 7.23):
  2. The supraorbital nerve passes through the supraorbital notch (foramen) and will be seen when the scalp is studied.
  1. The infraorbital nerve emerges from the infraorbital foramen to supply sensory innervation to the inferior eyelid, side of the nose, and upper lip. On the right side, use blunt dissection to define the borders of the levator labii superioris muscle. Transect the muscle close to the infraorbital margin and reflect it inferiorly to expose the infraorbital nerve (V2).
  2. In the midline, make an incision through the entire thickness of the lower lip, extending as far inferiorly as the mental protuberance. On the right side, make a second incision parallel to the first. The second incision should begin at the angle of the mouth and end at the margin of the mandible. Reflect the flap of lip inferiorly.
  3. Cut through the mucous membrane where it reflects from the lip to the gums. Use blunt dissection to peel the flap of lip from the bone and locate the mental foramen (L. mentum, chin). The mental foramen is located approximately 3 cm from the median plane.
  4. Observe that the mental nerve, artery, and vein emerge from the mental foramen. The mental nerve supplies sensory innervation to the lower lip and chin.
  5. There are several smaller branches of the trigeminal nerve that innervate the facial region (lacrimal, infratrochlear, zygomaticofacial, zygomaticotemporal, etc.). Do not dissect these branches. The auriculotemporal nerve (a branch of V3) will be dissected later.

IN THE CLINIC: Dental Anesthesia

Study the infraorbital foramen and infraorbital canal in the skull. For purposes of dental anesthesia, the infraorbital nerve may be infiltrated where it emerges from the infraorbital foramen. The needle is inserted through the oral mucosa deep to the upper lip and directed superiorly.

Dissection Review

 

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TUESDAY, MARCH 23 – FINISH FACE; SCALP

 

Scalp

Upon completion of this session, the student will be able to:

Define the scalp, its structural layers, muscles, nerves, and vessels.

Dissection Overview

The scalp consists of five layers that are firmly bound together (Fig. 7.26):

As an aid to memory, note that the first letters of the names of the five layers spell the word  S – C – A – L - P.

  

IN THE CLINIC: Scalp

The connective tissue layer of the scalp contains collagen fibers that attach to the external surface of the blood vessels. When a blood vessel of the scalp is cut, the connective tissue holds the lumen open, resulting in profuse bleeding.

If an infection occurs in the scalp, it can spread within the loose connective tissue layer. Therefore, the loose connective tissue layer is often called the “dangerous area.” From the “dangerous area,” the infection may pass into the cranial cavity through emissary veins.

 

The order of dissection will be as follows: The scalp will be reflected. The muscles of the scalp will be examined on the cut surface of the scalp.

 

Dissection Instructions

  1. These cuts should be made through the entire scalp and the scalpel should contact the bones of the calvaria.
  2. Refer to Figure 7.27 and make a midline cut from the nasion (C) to the vertex (A). Extend this cut to the external occipital protuberance (G).
  3. Make a cut in the coronal plane from the vertex (A) to the ear (D). Duplicate this cut on the opposite side of the head.
  4. Beginning at the vertex, use forceps to grasp one corner of the cut scalp and insert a chisel between the scalp and the calvaria. Use the chisel to loosen the scalp from the calvaria.
  1. Once the flap of scalp is raised, grasp the flap with both hands and pull it inferiorly.
  2. Reflect all four flaps of scalp down to the level that a hatband would occupy (Fig. 7.28). Do not detach the flaps.
  3. Examine the cut edge of the scalp and identify the occipitofrontalis muscle (Fig. 7.29). The inferior attachment of the occipital belly is the occipital bone and its superior attachment is the epicranial aponeurosis. The superior attachment of the frontal belly is the epicranial aponeurosis and its inferior attachment is the skin of the forehead and eyebrows. Both muscles are innervated by the facial nerve (VII).
  1. Pull the anterior scalp flap inferiorly to expose the supraorbital margin. Identify the supraorbital nerve and vessels where they exit the supraorbital notch and enter the deep surface of the scalp (Fig. 7.28).
  2. Use an illustration to observe that nerves and vessels are contained within the flaps of the scalp (Fig. 7.30). Note that the nerves and vessels enter the scalp from more inferior regions.
  3. On the lateral surface of the calvaria, note that the scalp has been separated from the fascia that covers the temporalis muscle (temporal muscle).

 

Dissection Review

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WEDNESDAY, MARCH 24 – INTERIOR OF SKULL; MENINGES OF BRAIN

 

 

Interior of the Skull

Upon completion of this session, the student will be able to:

1.     Identify the prominent landmarks on the internal surface of the skull base.

2.     Identify the major blood vessels of the brain, the specializations of cranial meninges, and cranial dural modifications.

3.     Identify the cranial nerves on the brain and their courses through the skull base.

4.     Identify the parts of the ventricular system and trace the flow of cerebrospinal fluid from production to reabsorption.

Dissection Overview

 

The bones of the calvaria provide a protective covering for the cerebral hemispheres. To view the internal features of the cranial cavity, the calvaria must be removed. In addition, a wedge of occipital bone will be removed to open the dissection field and make removal of the brain easier.

The order of dissection will be as follows: The scalp and temporalis muscle will be reflected inferiorly. The calvaria will be cut with a saw and removed. A wedge of occipital bone will be removed. The dura mater will be examined and then opened to reveal the arachnoid mater and pia mater.

 

Dissection Instructions

 

Removal of the Calvaria

  1. The cadaver should be in the supine position. Reflect the scalp inferiorly.
  2. Use a scalpel to detach the temporalis muscle from the calvaria and reflect the temporalis muscle inferiorly. Fold it down over the reflected scalp (Fig. 7.37).
  3. Observe the pericranium that covers the calvaria. Use a scalpel or chisel to scrape the bones of the calvaria clean of periosteum and muscle fibers.
  4. Place a rubber band around the circumference of the skull (Fig. 7.37, dashed line). Anteriorly, the rubber band should be about 2 cm superior to the supraorbital margin. Posteriorly, the rubber band should be about 2 cm superior to the external occipital protuberance. Use the rubber band as a guide to mark the circumference of the calvaria with a pencil line.
  1. Refer to a skull. Remove the calvaria of the skull and note that the bones of the calvaria have three layers:
  2. Use the electric autopsy saw to cut along the pencil line. The saw cut should pass through the outer lamina of the calvaria but not completely through the bone. Moist red bone indicates that the saw is within the diploλ. Be particularly careful when cutting the squamous part of the temporal bone, which is very thin. If you saw through the inner lamina, you may damage the underlying dura mater or the brain.
  3. While sawing, turn the body alternately from supine to prone and back to supine as you work your way around the skull. After making a complete circumferential cut, break the inner lamina of the calvaria by repeatedly inserting a chisel at a 45 degree angle (this oblique placement of the chisel will prevent you from damaging the brain) into the saw cut and striking the chisel gently with a mallet. Continue with this procedure until the calvaria can be pried loose using the Virchow skull cracker (supplied by your instructor).
  4. Remove the calvaria by prying it from the dura mater with the handle of a forceps or a chisel blade. Work from anterior to posterior and do not use more force than is necessary. Violent pulling may result in tearing of the dura and damage to the brain.

 

Removal of a Wedge of Occipital Bone

  1. Place the cadaver in the prone position and refer to Figure 7.38.
  2. Use a scalpel to detach the semispinalis capitis muscle, splenius capitis muscle, obliquus capitis superior muscle, and rectus capitis posterior major and minor muscles from the occipital bone.
  3. Identify the posterior atlanto-occipital membrane, which spans the interval between the atlas (C1) and the occipital bone. Use a scalpel to incise the posterior atlanto-occipital membrane transversely from the left vertebral artery to the right vertebral artery. Preserve the vertebral arteries.
  4. Use a scalpel or chisel to scrape the occipital bone clean of remaining muscle fibers and pericranium.
  5. Review the following landmarks on a skull (Fig. 7.38):
  6. On the skull, note the point where the lambdoid suture meets the saw cut where the calvaria was removed. Transfer this point to the cadaver specimen and mark the location with a pencil (Fig. 7.39, point A).
  1. On the skull, examine the internal surface of the occipital bone and identify (Fig. 7.40):
  1. On the external surface of the skull, identify the lateral margin of the foramen magnum and transfer this point to the cadaver specimen (Fig. 7.39, point B). On the right and left sides of the cadaver, connect points A and B with a pencil line to define the wedge of occipital bone that will be removed in the cadaver.
  1. Use a saw to cut along the pencil lines. As in the removal of the calvaria, do not cut through the inner lamina of compact bone. Extend the saw cut into the foramen magnum but preserve the vertebral arteries. Loosen the wedge of bone with chisel and mallet and remove it, leaving the dura mater intact (Fig. 7.41).

Cranial Meninges

  1. The brain is covered with three membranes called meninges (Gr. meninx, membrane). From outside to inside they are (Fig. 7.42):
  2. The dura mater (L. dura mater, hard mother) consists of two layers, an external periosteal layer and an internal meningeal layer (Fig. 7.42). The two dural layers are indistinguishable except where they separate to enclose the dural venous sinuses.
  3. Identify the superior sagittal sinus and the right and left transverse sinuses (Fig. 7.41).
  1. Use scissors to make a longitudinal incision in the superior sagittal sinus (Fig. 7.43) and verify that:
  2. Examine the surface of the dura mater that covers the cerebral hemispheres and observe the branches of the middle meningeal artery. The middle meningeal artery supplies the dura mater and adjacent calvaria. Note that the anterior branch of the middle meningeal artery crosses the inner surface of the pterion, where it may tunnel through the bone. Fractures through the pterion may result in tearing of the middle meningeal artery.
  3. Examine the inner surface of the removed calvaria. Identify the following features:
  4. Use scissors to make a parasagittal cut through the dura mater about 2 cm lateral to the midline (Fig. 7.43). Cut only the dura mater, not the arachnoid mater. This cut should be lateral and parallel to the lateral edge of the superior sagittal sinus. Extend the cut to the frontal bone anteriorly and to the transverse sinus posteriorly. Duplicate the parasagittal cut on the opposite side of the cadaver.
  1. Make a coronal cut through the dura mater from the midpoint of the parasagittal cut (near the vertex) to just above the ear (Fig. 7.43). Repeat on the opposite side of the midline.
  2. The result of these cuts is a median strip of dura mater containing the superior sagittal sinus and four flaps of dura mater that are similar in position to the scalp flaps (Fig. 7.43). Fold the dural flaps inferiorly over the cut edge of the skull. Use scissors to detach any small adhesions or blood vessels that constrain the flaps.
  3. Observe the arachnoid mater (Gr. arachnoeides, like a cobweb—in reference to the spider web–like connective tissue strands in the subarachnoid space). The arachnoid mater loosely covers the brain and spans across the fissures and sulci. In the living person, the arachnoid mater is closely applied to the internal meningeal layer of the dura mater with no space between (Fig. 7.42).
  4. Observe the cerebral veins that are visible through the arachnoid mater. The cerebral veins empty into the superior sagittal sinus. At the point where the cerebral veins enter the sinus, they may be torn in cases of head trauma.
  5. Use scissors to make a small cut (2.5 cm) through the arachnoid mater over the lateral surface of the brain. Use a probe to elevate the arachnoid mater and observe the subarachnoid space. In the living person, the subarachnoid space is a real space that contains cerebrospinal fluid.
  6. Through the opening in the arachnoid mater, observe the pia mater (L. pia mater, tender mother) on the surface of the brain. The pia mater faithfully follows the contours of the brain, passing into all sulci and fissures. The pia mater cannot be removed from the surface of the brain.

IN THE CLINIC: Epidural Hematoma

When the middle meningeal artery is torn in a head injury, blood accumulates between the skull and the dura mater (epidural hematoma).

IN THE CLINIC: Subdural Hematoma

As a complication of head injury, cerebral veins may bleed into the potential space between the dura mater and the arachnoid mater. When this happens, the blood accumulates between the dura mater and arachnoid mater (a “subdural space” is created), and this condition is called a subdural hematoma.

Dissection Review

 

RETURN TO TABLE OF CONTENTS

 

 

 

 

 

MONDAY, MARCH 29 – REMOVAL OF THE BRAIN AND THE CRANIAL FOSSAE

 

Removal of the Brain and the Cranial Fossae

 

Dissection Overview

 

The internal meningeal layer of the dura mater forms inwardly projecting folds (dural infoldings) that serve as incomplete partitions of the cranial cavity. Three of these folds (cerebral falx, cerebellar tentorium, and cerebellar falx) extend inward between parts of the brain. These infoldings must first be cut before the brain can be removed.

The order of dissection will be as follows: The brain will be removed intact, along with the arachnoid mater and pia mater. The dura mater will be left in the cranial cavity, where the dural infoldings will be studied.

 

Dissection Instructions

Removal of the Brain

  1. In the midline, use your fingers to gently retract one cerebral hemisphere 1 or 2 cm laterally and observe the cerebral falx (L. falx, sickle) between the cerebral hemispheres (Fig. 7.45). The cerebral falx is attached to the crista galli at its anterior end and the cerebellar tentorium at its posterior end.
  2. Use your hand to gently lift the frontal lobes (anterior part of the brain) and use scissors to cut the cerebral falx where it is attached to the crista galli.
  3. Use scissors to cut the cerebral veins where they enter the superior sagittal sinus (Fig. 7.43) so that the veins will remain on the surface of the brain. Grasp the cerebral falx near the crista galli and pull it superiorly and posteriorly from between the cerebral hemispheres. At its posterior end, the cerebral falx will remain attached to the cerebellar tentorium.
  4. On the right side, gently lift the occipital lobe (posterior part of brain) and observe the cerebellar tentorium. Beginning anteriorly, use a scalpel to cut the cerebellar tentorium as close to bone as possible. Sever the cerebellar tentorium from the posterior clinoid process and then from the superior border of the petrous part of the temporal bone (Fig 7.44). The cut should continue to the posterolateral end of the superior border of the petrous part of the temporal bone, near the groove for the sigmoid sinus. Repeat the cut on the left side of the cadaver.
  1. Pull the cerebral falx and cerebellar tentorium posteriorly from between the cerebral hemispheres and cerebellum. This procedure will tear the great cerebral vein (Fig 7.46).
  2. With the dural infoldings detached, the brain may be gently moved to expose the cranial nerves and vessels that are located on its inferior surface.
  1. Use your fingers to gently elevate the frontal lobes. Use a probe to lift the olfactory bulb from the cribriform plate on each side of the crista galli (Fig 7.50).
  2. Use a scalpel to cut the following structures bilaterally: optic nerve, internal carotid artery, and oculomotor nerve (Fig 7.50). Cut the stalk of the pituitary gland in the midline.
  3. Raise the brain slightly higher and cut the following structures bilaterally: trochlear nerve, trigeminal nerve, and abducent nerve (Fig 7.50).
  4. Elevate the cerebrum and brainstem still further and cut the following structures bilaterally: facial and vestibulocochlear nerves near the internal acoustic meatus; glossopharyngeal, vagus, and accessory nerves near the jugular foramen; and hypoglossal nerve near the hypoglossal canal (Fig 7.50).
  5. Sever the two vertebral arteries where they enter the skull through the foramen magnum. Use a scalpel to cut the cervical spinal cord in cervical vertebral canal between the atlas and the occipital bone.
  6. Support the brain with the palm of one hand under the frontal lobes and your fingers extending down the ventral surface of the brainstem. Insert the tip of your middle finger into the cut that was made across the cervical spinal cord to support the brainstem and cerebellum. Roll the brain, brainstem, and cerebellum posteriorly and out of the cranial cavity in one piece.
  7. The brain with an identifying tag indicating your table number should be placed in a bucket of preservative. The faculty will evaluate the brain for the intactness of the cranial nerves and vasculature. Winners will be notified and invited to a celebratory dinner at Dr. Giffin’s house.

Dural Infoldings and Dural Venous Sinuses

 

Dissection Overview

The two layers of the dura mater separate in several locations to form dural venous sinuses. The dural venous sinuses collect venous drainage from the brain and conduct it out of the cranial cavity.  The order of dissection will be as follows: The dura mater will be repositioned to recreate its three-dimensional morphology during life. The infoldings of the dura mater and the associated dural venous sinuses will be identified.

 

Dissection Instructions

 

Dural Infoldings

 

Return the dura mater to its correct anatomical position.

  1. On the right side of the head, open the two flaps of dura mater and identify the cerebral falx (falx cerebri) (Fig 7.46). In the living person, the cerebral falx lies between the cerebral hemispheres. The cerebral falx is attached to the crista galli, the calvaria on both sides of the groove for the superior sagittal sinus, and the cerebellar tentorium.
  2. Identify the cerebellar tentorium (tentorium cerebelli; L. tentorium, tent) (Fig. 7.45). The cerebellar tentorium is attached to the clinoid processes of the sphenoid bone, the superior border of the petrous portion of the temporal bone, and the occipital bone on both sides of the groove for the transverse sinus. The opening in the cerebellar tentorium is called the tentorial notch (tentorial incisure), and the brainstem passes through it. In the living person, the cerebellar tentorium is between the cerebral hemispheres and the cerebellum.
  3. Identify the cerebellar falx (falx cerebelli), which is located inferior to the cerebellar tentorium in the midline (Fig 7.46). Note that the cerebellar falx is attached to the inner surface of the occipital bone and that it is located between the cerebellar hemispheres.

Dural Venous Sinuses

 

Review the position of the superior sagittal sinus (Fig 7.46). Note that the superior sagittal sinus begins near the crista galli and ends near the cerebellar tentorium by draining into the confluence of sinuses.

  1. Identify the inferior sagittal sinus, which is in the inferior margin of the cerebral falx (Fig 7.46). The inferior sagittal sinus begins anteriorly and ends near the cerebellar tentorium by draining into the straight sinus. Note that the inferior sagittal sinus is much smaller in diameter than the superior sagittal sinus.
  2. The straight sinus is located in the line of junction of the cerebral falx and the cerebellar tentorium. At its anterior end, the straight sinus receives the inferior sagittal sinus and the great cerebral vein. The straight sinus drains into the confluence of sinuses.
  3. Review the position of the transverse sinuses (right and left). Each transverse sinus carries venous blood from the confluence of sinuses to the sigmoid sinus. Use a scalpel to open the lumen of the transverse sinus.
  4. Identify the sigmoid sinus. The sigmoid sinus begins at the lateral end of the transverse sinus and ends at the jugular foramen. Use a scalpel to open the lumen of the sigmoid sinus and trace it to the jugular foramen. The internal jugular vein is formed at the external surface of the jugular foramen.
  5. Observe the floor of the cranial cavity. Note that the dura mater covers all of the bones and provides openings through which the cranial nerves pass. There are small dural venous sinuses located between the layers of the dura mater in the floor of the cranial cavity. Use an atlas illustration to study the following dural venous sinuses:

Dissection Review

Cranial Fossae

 

Dissection Overview

The order of dissection will be as follows: The bones of the floor of the cranial cavity will be studied and the boundaries of the cranial fossae will be identified. The vessels and the nerves of each cranial fossa will be studied. Because the floor of the cranial cavity is covered by dura mater, the dissection is much easier if a skull is held beside the cadaver specimen during dissection to permit direct observation of the foramina.

 

Skeleton of the Cranial Base

Use a skull to identify (Fig 7.49):

Identify the foramen lacerum, which is formed by portions of the greater wing of the sphenoid bone and the temporal bone.  The anterior cranial fossa is separated from the middle cranial fossa by the right and left sphenoidal crests and the sphenoidal limbus. The middle cranial fossa is separated from the posterior cranial fossa by the superior border of the petrous part of the right and left temporal bones and the dorsum sellae. The cerebellar tentorium is attached to the superior border of the petrous part of the temporal bone and it forms the roof of the posterior cranial fossa.

 

Dissection Instructions

 

Anterior Cranial Fossa

  1. On the right side of the cadaver only, use a probe to loosen the dura mater along the cut edge of the frontal bone. Grasp the dura mater with your fingers and pull it posteriorly as far as the lesser wing of the sphenoid bone. Use scissors to detach the dura mater along the sphenoidal crest and along the midline and place it in the tissue container.
  2. Note that the sphenoparietal venous sinus is located along the sphenoidal crest and that its lumen may now be visible where you detached the dura mater.
  3. Identify the three bones that participate in the formation of the anterior cranial fossa: sphenoid bone, ethmoid bone, and orbital part of the frontal bone (Fig 7.49). Note that the orbital part of the frontal bone forms the roof of the orbit.
  4. Before the brain was removed, the cerebral falx was attached to the crista galli and the frontal lobe of the brain rested on the orbital part of the frontal bone. The olfactory bulb rested on the cribriform plate and the fibers of the olfactory nerve (I) passed through the openings of the cribriform plate to enter the nasal cavity (Fig 7.50).

Middle Cranial Fossa

 

Recall that the middle cranial fossa contains the temporal lobe of the brain.

  1. Observe the dura mater that covers the floor of the middle cranial fossa. The dura mater hides all of the openings in the skull and the nerves and vessels that pass through them (Fig 7.50).
  2. Identify the middle meningeal artery that can be seen through the dura mater (Fig 7.50). It appears as a dark line extending laterally from the deepest point of the middle cranial fossa. The middle meningeal artery enters the middle cranial fossa by passing through the foramen spinosum.
  3. ON THE RIGHT SIDE OF THE CADAVER ONLY, grasp the dura mater along the sphenoidal crest and peel it posteriorly as far as the superior border of the petrous part of the temporal bone. Note that the middle meningeal artery adheres to the external surface of the dura mater. Use a probe to tease the proximal part of middle meningeal artery away from the dura mater and leave it in the skull.
  1. ON THE RIGHT SIDE OF THE CADAVER ONLY, use scissors to detach the dura mater along the superior border of the petrous part of the temporal bone and place it in the tissue container. Do not cut the cranial nerves that cross the anterior end of the superior border of the petrous part of the temporal bone (oculomotor, trigeminal, trochlear, and abducent). Note that the lumen of the superior petrosal sinus can be seen along the line of the cut (Fig 7.50).
  2. Observe that the floor of the middle cranial fossa is formed by two bones: sphenoid and temporal (Fig 7.49).
  3. Identify the optic nerve (II) (Fig 7.50). The optic nerve passes through the optic canal to enter the orbit. The optic nerve is surrounded by a sleeve of dura mater as it exits the middle cranial fossa.
  4. Use a probe to identify the superior orbital fissure that is located inferior to the lesser wing of the sphenoid bone (Fig 7.49). Three cranial nerves and part of a fourth exit the middle cranial fossa by passing through the superior orbital fissure:
  5. Use a probe to clean the nerves that pass through the superior orbital fissure. Note that three of these nerves are located within the lateral wall of the cavernous sinus (III, IV, V1) and one is within the cavernous sinus (VI) (Fig 7.51).
  6. Identify the trigeminal nerve (V) (Fig 7.50). It is the largest cranial nerve and is easily found where it crosses the superior border of the petrous part of the temporal bone.
  7. Follow the trigeminal nerve anteriorly and identify the trigeminal ganglion. Use a probe to define the three divisions (nerves) that arise from the anterior border of the trigeminal ganglion (ophthalmic [V1], maxillary [V2], and mandibular [V3]). Note that these three divisions are named according to their region of distribution and are numbered from superior to inferior as they arise from the trigeminal ganglion.
  8. Identify the maxillary division of the trigeminal nerve (V2) (Fig 7.50), and follow it anteriorly to the foramen rotundum (Fig 7.49), where it exits the middle cranial fossa. The maxillary division courses within the lateral wall of the cavernous sinus just inferior to the ophthalmic division of the trigeminal nerve (V1) (Fig 7.51).
  9. Identify the mandibular division of the trigeminal nerve (V3) (Fig 7.50) and follow it inferiorly to the foramen ovale (Fig 7.49), which is where it exits the middle cranial fossa.
  10. Return to the area of the cavernous sinus and use a probe to retract the cranial nerves. Identify the internal carotid artery (Fig 7.50). The internal carotid artery enters the cranial cavity by passing through the carotid canal. It makes an S-shaped bend in the cavernous sinus and emerges near the optic nerve. Cranial nerves III, IV, V1, V2, and VI cross the lateral side of the internal carotid artery. Among this group of nerves, the abducent nerve (VI) is most closely related to the internal carotid artery (Fig 7.51).
  1. Identify the region of the hypophyseal fossa (Fig 7.49). The hypophyseal fossa is covered by the sellar diaphragm (diaphragma sellae), which is a dural infolding (Fig 7.51). The stalk of the pituitary gland passes through an opening in the sellar diaphragm. The pituitary gland is still contained within the hypophyseal fossa.
  2. Anterior and posterior to the stalk of the pituitary gland are two small dural venous sinuses called the anterior and posterior intercavernous sinuses (Fig 7.50). The intercavernous sinuses connect the right and left cavernous sinuses across the midline. Do not attempt to dissect the intercavernous sinuses.
  3. Use an atlas illustration to identify all of the veins and venous sinuses that drain into or out of the cavernous sinus.

 

IN THE CLINIC: Cavernous Sinus

In fractures of the base of the skull, the internal carotid artery may rupture within the cavernous sinus. The release of arterial blood into the cavernous sinus creates an abnormal reflux of blood from the cavernous sinus into the ophthalmic veins. As a result, the orbit is engorged and the eyeball is protruded and is pulsating in synchrony with the radial pulse (pulsating exophthalmos).

Posterior Cranial Fossa

  1. Recall that the posterior cranial fossa contains the cerebellum and the brainstem. At the foramen magnum, the brainstem becomes continuous with the cervical spinal cord. The features of the posterior cranial fossa will be studied with the dura mater intact.
  2. Identify the facial nerve (VII) and the vestibulocochlear nerve (VIII) where they enter the internal acoustic meatus (Fig 7.50). Do not follow them into the bone at this time.
  3. The jugular foramen is inferior to the internal acoustic meatus (Fig 7.49). Identify the rootlets of the glossopharyngeal nerve (IX), the vagus nerve (X), and the accessory nerve (XI) where they enter the jugular foramen. Because cranial nerves IX and X are formed by rootlets, it is difficult to distinguish one nerve from the other as they enter the jugular foramen. However, the cervical root of the accessory nerve can be positively identified because it enters the posterior cranial fossa through the foramen magnum and crosses the inner surface of the occipital bone (Fig 7.50).
  4. Identify the hypoglossal nerve (XII) where it enters the hypoglossal canal (Fig 7.50).
  5. Review the course of the transverse sinus and sigmoid sinus. Observe that the sigmoid sinus ends at the jugular foramen posterior to the exit point of cranial nerves IX, X, and XI.
  6. On the left (undissected) side of the cranial cavity, identify the cranial nerves in order from anterior to posterior (Fig 7.50).

Dissection Review

 

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THURSDAY, APRIL 8; FRIDAY, APRIL 9 – ORBIT AND CONTENTS

 

 

Orbit

 

Upon completion of this session, the student will be able to:

1.     Identify the prominent bony features of the orbit with included foramina and fissures.

2.     Describe the components of the eyelids with associated muscles, tarsal glands, connective tissue fascia and conjunctiva.

3.     Identify the extraocular muscles, their function and innervation.

4.     Identify all sensory, motor and autonomic nerves of the orbit and trace their routes to and within the orbit.

5.     Identify branches of ophthalmic arteries and veins.

 

Dissection Overview

The orbit contains the eyeball and extraocular muscles. The eyeball is about 2.5 cm in diameter and occupies the anterior half of the orbit. The posterior half of the orbit contains fat, extraocular muscles, branches of cranial nerves, and blood vessels. Some vessels and nerves pass through the orbit to reach the scalp and face.

 

The order of dissection will be as follows: The bones of the orbit will be studied. On the right side only, the floor of the anterior cranial fossa will be removed and the right orbit will be dissected from a superior approach. Cranial nerves III, IV, V1, and VI will be followed through the superior orbital fissure into the orbit and the extraocular muscles will be identified. On the left side only, the anatomy of the eyelid will be studied. The attachments of the extraocular muscles will be studied.

 

Skeleton of the Orbit

The bones of the orbit form a four-sided pyramid. The base of the pyramid is the orbital margin and the apex of the pyramid is the optic canal. Viewed from above, the medial walls of the two orbits are parallel to each other and about 2.5 cm apart. The lateral walls of the two orbits form a right angle to each other.

 

Refer to a skull and identify the bones that participate in the formation of the walls of the orbit (Fig 7.52):

On the medial wall of the orbit, identify the anterior and posterior ethmoidal foramina.

 

Identify the inferior orbital fissure, which is a gap between the maxilla and the greater wing of the sphenoid bone. Note that the lateral wall of the orbit is stout and strong but the part of the ethmoid bone that forms the medial wall is paper thin and for this reason it is called the lamina papyracea. Examine a coronal section through the orbit and note the following relationships (Fig 7.53):

The bones of the orbit are lined with periosteum called periorbita. At the optic canal and the superior orbital fissure, the periorbita is continuous with the dura mater of the middle cranial fossa.

 

Surface Anatomy of the Eyeball, Eyelids, and Lacrimal Apparatus (do this some other time when you are not in the Gross Anatomy lab)

Use a mirror or recruit the assistance of your lab partner to inspect the living eye.

 

Identify:

In the medial angle of the eye, observe:

Evert the lower lid slightly and observe:

Use an illustration to study the following features and relate them to the living eye:

Dissection Instructions

Eyelid and Lacrimal Apparatus

  1. Dissect the eyelid and lacrimal gland only in the left eye.
  2. Review the attachments of the orbicularis oculi muscle. Use a probe to raise the lateral part of the orbital portion of the orbicularis oculi muscle and reflect the muscle medially.
  3. Raise the thin palpebral portion of the orbicularis oculi muscle off the underlying tarsal plate and reflect the muscle medially.
  4. The orbital septum is a sheet of connective tissue that is attached to the periosteum at the margin of the orbit and to the tarsal plates (Fig 7.54 and Fig 7.55). The orbital septum separates the superficial fascia of the face from the contents of the orbit.
  5. Identify the tarsal plates, which give shape to the eyelids (Fig 7.54). Tarsal glands are embedded in the posterior surface of each tarsal plate. Tarsal glands drain by small orifices that are located posterior to the eyelashes. Tarsal glands secrete an oily substance onto the margin of the eyelid that prevents the overflow of lacrimal fluid (tears).
  6. The lacrimal gland occupies the lacrimal fossa in the frontal bone (Fig 7.54). To find the lacrimal gland, use a scalpel to cut through the orbital septum adjacent to the orbital margin in the superolateral quadrant of the left orbit. Pass a probe through the incision and free the lacrimal gland from the lacrimal fossa. Note that the lacrimal gland drains into the superior conjunctival fornix by 6 to 10 short ducts (Fig 7.56).
  7. Use a skull to identify the lacrimal groove at the medial side of the orbital margin. Observe that the anterior lacrimal crest of the maxilla forms the anterior border of the lacrimal groove. The medial palpebral ligament is attached to the anterior lacrimal crest and the lacrimal sac lies posterior to the medial palpebral ligament (Fig 7.54).
  1. Two lacrimal canaliculi drain lacrimal fluid from the medial angle of the eye into the lacrimal sac. The nasolacrimal duct extends inferiorly from the lacrimal sac and enters the inferior meatus of the nasal cavity (Fig 7.56).
  2. Lacrimal fluid flows from the lacrimal gland across the eyeball to the medial angle of the eye.

o   During crying, excess lacrimal fluid cannot be emptied through the lacrimal canaliculi and tears overflow the lower eyelids. Increased drainage of tears into the nasal cavity results in sniffling, which is characteristic of crying.

 

IN THE CLINIC: Tarsal Glands

If the duct of a tarsal gland becomes obstructed, a chalazion (cyst) will develop. A chalazion will be located between the tarsal plate and the conjunctiva. In contrast, a hordeolum (stye) is the inflammation of a sebaceous gland associated with the follicle of an eyelash.

Dissection of the Orbit

  1. Dissect the orbits from the superior approach. Wear eye protection for all steps that require the use of bone cutters.
  2. In the floor of the anterior cranial fossa, tap the orbital part of the frontal bone with the handle of a chisel until the bone cracks. Use bone cutters to pick out the bone fragments and enlarge the opening in the roof of the orbit. Remove the roof of the orbit as far anteriorly as the superior orbital margin.
  3. The frontal bone contains the frontal sinus that may extend into the roof of the orbit. Medially, the ethmoidal cells may extend into the roof of the orbit. If either situation occurs in your cadaver, you must remove the mucous membrane that lines the sinus and remove a second layer of thin bone to open the orbit.
  4. Identify the membrane just inferior to the roof of the orbit. This is the periorbita, which lines the bones of the orbit.
  5. Push a probe posteriorly between the roof of the orbit and the periorbita. The probe should pass inferior to the lesser wing of the sphenoid bone, through the superior orbital fissure, and into the middle cranial fossa. Use the probe to break the lesser wing of the sphenoid bone.
  6. Use bone cutters to remove the fragments of the lesser wing of the sphenoid bone. Chip away the roof of the optic canal and remove the anterior clinoid process (Fig 7.57).
  7. Examine the periorbita and note that the frontal nerve may be visible through it. Use scissors to incise the periorbita from the apex of the orbit to the midpoint of the superior orbital margin. Use forceps to lift the periorbita off deeper structures and make a transverse incision through the periorbita, close to the superior orbital margin. Use a probe to tease open the flaps of periorbita and use scissors to remove them.
  8. The use of a fine probe and fine forceps is recommended from this point onward in the dissection.
  9. Three nerves enter the apex of the orbit by passing superior to the extraocular muscles:
  10. While preserving the nerves, use forceps to pick out lobules of fat and expose the superior surface of the levator palpebrae superioris muscle (Fig 7.55 and Fig 7.57). The levator palpebrae superioris muscle attaches to the upper eyelid, which it elevates.
  11. On the medial side of the orbit, identify the superior oblique muscle and trace it anteriorly (Fig 7.58). Observe that the tendon of the superior oblique muscle passes through the trochlea (L. trochlea, pulley), bends at an acute angle, and attaches to the posterolateral portion of the eyeball.
  12. On the lateral side of the orbit, identify the lateral rectus muscle (Fig 7.58). The lateral rectus muscle arises by two heads from the common tendinous ring. The common tendinous ring surrounds the optic canal and part of the superior orbital fissure, and it is the posterior attachment of the four rectus muscles. The optic nerve (II), nasociliary nerve, oculomotor nerve (III), and abducent nerve (VI) pass through the common tendinous ring.

 

PERFORM THE FOLLOWING STEPS ON THE RIGHT SIDE OF THE CADAVER ONLY

  1. Transect the levator palpebrae superioris muscle as far anteriorly as possible and reflect it posteriorly.
  2. Identify the superior rectus muscle that lies immediately inferior to the levator palpebrae superioris muscle (Fig 7.55 and Fig 7.57). Clean the superior rectus muscle and observe that it is attached to the eyeball by a thin, broad tendon.
  3. The superior ophthalmic vein begins at the inner angle of the orbit which communicates with the angular vein; it pursues the same course as the ophthalmic artery, and receives tributaries corresponding to the branches of that vessel. Forming a short single trunk, it passes between the two heads of the lateral rectus muscle and through the medial part of the superior orbital fissure, and ends in the cavernous sinus.
  4. Transect the superior rectus muscle close to the eyeball and reflect it posteriorly (Fig 7.58). Note that a branch of the superior division of the oculomotor nerve (III) reaches the inferior surface of the superior rectus muscle. A branch of the superior division passes around the medial side of the superior rectus muscle to innervate the levator palpebrae superioris muscle.
  1. Use scissors to cut the common tendinous ring between the attachments of the superior rectus and lateral rectus muscles. All structures passing through the common tendinous ring are now exposed.
  2. Identify the abducent nerve (VI). The abducent nerve passes between the two heads of the lateral rectus muscle, turns laterally, and enters the medial surface of the lateral rectus muscle. Find the abducent nerve on the medial surface of the lateral rectus muscle near the apex of the orbit (Fig 7.58).
  3. Identify the nasociliary nerve, which is a branch of V1 (Fig 7.58). Trace the nasociliary nerve through the orbit and note that it is much smaller than the frontal nerve. The nasociliary nerve crosses superior to the optic nerve and gives off several long ciliary nerves to the posterior part of the eyeball.
  4. Follow the nasociliary nerve toward the medial wall of the orbit. Use forceps to pick out the fat that fills the intervals between muscles, nerves, and vessels.
  5. Identify the anterior ethmoidal nerve, which is a small branch of the nasociliary nerve that passes through the anterior ethmoidal foramen. The anterior ethmoidal nerve supplies part of the mucous membrane in the nasal cavity. Its terminal branch is the external nasal nerve that innervates the skin at the tip of the nose.
  6. In the middle cranial fossa, identify the oculomotor nerve within the lateral wall of the cavernous sinus. Follow the oculomotor nerve to the superior orbital fissure where it branches into two divisions:
  7. The ciliary ganglion is a parasympathetic ganglion located between the optic nerve and the lateral rectus muscle. It is approximately 2 mm in diameter and is located about 1 cm anterior to the apex of the orbit (Fig 7.58). Note that short ciliary nerves connect the ciliary ganglion to the posterior surface of the eyeball. Study the autonomic function of the ciliary ganglion. THIS GANGLION MAY BE DIFFICULT TO SEE RESEMBLING A LITTLE WAD OF FAT. DO NOT GROW OLD LOOKING FOR IT!
  8. Use an atlas illustration to study the course of the superior ophthalmic vein in the orbit. At the medial angle of the eye, the superior ophthalmic vein anastomoses with the angular vein, which is a tributary of the facial vein.
  9. Identify the optic nerve (II) (Fig 7.58). The optic “nerve” is actually a brain tract and it is surrounded by the three meningeal layers: dura mater, arachnoid mater, and pia mater.
  10. Identify the ophthalmic artery where it branches from the internal carotid artery (Fig 7.50). In its course through the orbit, note that the ophthalmic artery usually crosses superior to the optic nerve and reaches the medial wall of the orbit. Use a probe to gently tease out the posterior ciliary arteries that supply the eyeball.
  11. The medial rectus, inferior rectus, and inferior oblique muscles are not easily seen from the superior approach. Carefully push aside the dissected structures to catch a glimpse of these muscles.

IN THE CLINIC; Ophthalmic Veins

Anastomoses between the angular vein and the superior and inferior ophthalmic veins are of clinical importance. Infections of the upper lip, cheeks, and forehead may spread through the facial and angular veins into the ophthalmic veins and then into the cavernous sinus. Thrombosis of the cavernous sinus may result, leading to involvement of the abducent nerve and dysfunction of the lateral rectus muscle.

 

 

Dissection Review

 

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